Conventionally, there is a tire air pressure warning device in a dynamic loaded radius system, which utilizes a fact that a tire rolling radius while a vehicle is running is proportional to air pressure in the tire to detect a decrease in tire air pressure based upon the tire rolling radius of each wheel, or more specifically, the difference in rotational speed between right and left wheels, and outputs a warning. In this case, even if both the right and left wheels have a proper pressure with no decrease in air pressure, the turning radii of the right and left wheels of the vehicle differ while the vehicle is turning, wherein a turning radius of the outer turning wheel is larger than that of the inner turning wheel. Therefore, the speed of the outer turning wheel becomes faster than that of the inner turning wheel, and furthermore, the wheel radius of the outer turning side becomes smaller than the wheel radius of the inner turning side due to the load shift of the vehicle while turning. Thus, a correct tire air pressure cannot be detected. Accordingly, an art that determines whether a vehicle is running straight, and determines an air pressure decrease based upon a measurement value during straight running is supposed.
Examples of such art include, for example, an art disclosed in Japanese Patent Laid-Open Publication No. HEI7-172121, that detects a vehicle turning state inappropriate to an air pressure determination using a steering sensor, and an art disclosed in Japanese Patent Laid-Open Publication No. HEI8-282222, that uses an average value of the right and left wheel speed ratio within a predetermined period as a reference and determines that the vehicle is turning when a predetermined difference occurs in relation to the average value. However, the former requires other sensors in addition to the wheel speed sensor, such as a steering sensor and depending on the case, a lateral G sensor and yaw rate sensor, which in turn increases cost. Furthermore, in the latter, for example, the average value itself becomes an inappropriate value for indicating an intermediate state between straight running and turning, as can be seen clearly on an oval course or the like where straight running and turning in a fixed direction are repeated, instead of an appropriate value indicating an average value of straight running. Thus, when this value is used as a reference value to determine straight running, the intended effect cannot be obtained.